Coding Challenge Solutions

Use a hash map to store complements as you iterate, achieving O(n) time instead of the brute-force O(n²) nested loop approach.

Push opening brackets onto a stack and pop when a matching closing bracket is found. If the stack is empty at the end, the string is valid.

Compare heads of both lists and recursively (or iteratively) attach the smaller node, building the merged list in sorted order.

Traverse left subtree, visit the node, then traverse right. Can be done recursively or iteratively with an explicit stack.

Track the minimum price seen so far and compute the max profit at each step. A single pass gives O(n) time and O(1) space.

Use Floyd's tortoise and hare algorithm — a slow and fast pointer. If they meet, there's a cycle; if fast reaches null, there isn't.